What is the current value of the scale factor?

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The current value of the scale factor, R₀, is defined as 1, a dimensionless number that simplifies cosmological calculations. This normalization allows for consistent scaling of the universe's expansion over time. The scale factor a(t) is expressed as R(t)/R₀, which effectively becomes a(t) = R(t) when R₀ is set to 1. This definition does not alter the underlying physics, as it is an arbitrary choice made for convenience. The discussion raises questions about the necessity of defining the scale factor when it appears to merely rename R(t).
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Do cosmologists know what is the current value of the scale factor? I know we have a current value for the Hubble constant H, and I know H=a'/a. But what about the value of a?

Thanks!
 
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cristo said:
Usually, the scale factor is normalised so that it takes the value a(t_0)=1 at present times. See this thread https://www.physicsforums.com/showthread.php?t=222284, for example.

What is the current value of the scale factor(R₀)?

a= R(t)/R₀

Give me the number.

Thanks
 
R0= 1, which is a dimensionless number. Why? Because I define it as 1. I can define it as any present number I want to, and it will not change the physics. I can arbitrarily define R at one time only; the value at all other times will scale with my choice of R at the single time I selected.
 
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Yeah, the scale factor is traditionally set to unity at the current time. In other words, a(t=2011 A.D.) = 1. Then you can apply the cosmology of your choice (e.g. flat universe with cosmological constant) and answer questions such as how old was the universe when things were 1000 times closer together: a(time in past) / a (now) = 1 / 1000 and solve for the time in the past.
 
BillSaltLake said:
R0= 1, which is a dimensionless number. Why? Because I define it as 1. I can define it as any present number I want to, and it will not change the physics. I can arbitrarily define R at one time only; the value at all other times will scale with my choice of R at the single time I selected.

Thanks, but here appears another question. So why do we define a=R/R0 at all, when R0=1? It means a(t)=R(t), and it is just changing names, isn't it?!
 
https://en.wikipedia.org/wiki/Recombination_(cosmology) Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in context of the calculation of the observable universe radius, where the time integral of the inverse of the scale factor is multiplied by the constant speed of light ##c##.
The formal paper is here. The Rutgers University news has published a story about an image being closely examined at their New Brunswick campus. Here is an excerpt: Computer modeling of the gravitational lens by Keeton and Eid showed that the four visible foreground galaxies causing the gravitational bending couldn’t explain the details of the five-image pattern. Only with the addition of a large, invisible mass, in this case, a dark matter halo, could the model match the observations...
Hi, I’m pretty new to cosmology and I’m trying to get my head around the Big Bang and the potential infinite extent of the universe as a whole. There’s lots of misleading info out there but this forum and a few others have helped me and I just wanted to check I have the right idea. The Big Bang was the creation of space and time. At this instant t=0 space was infinite in size but the scale factor was zero. I’m picturing it (hopefully correctly) like an excel spreadsheet with infinite...

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